Remote control apparatus



Jan. 13, 1959 E. K. WAGNER REMOTE CONTROL APPARATUS Filed Aug. 9, 1954 My -r-ronloevf Jan. 13, 1959 E. K. WAGNER REMOTE CONTROL APPARATUS 2Sheets-Sheet 2 Filed Aug. 9, 1954 NVB/@TQM eqlmv' @.mnef

Mfg/M zu CAT-roma??? United States 1 REMTE CNTRL APPARATUS Elmer K.Wagner, Rockford, lll., assigner to The Ingersoll Milling MachineCompany, Rockford, ill., a corporation of Illinois Application August 9,1954, Serial No. @43,53%

12 Claims. (Cl. Salti-mln) This invention relates generally toelectrical apparatus in which the energization of a plurality of loaddevices at an operating station is controlled by the selectivemanipulation of elements disposed at a remotely located control stationand in which the number of conductors required to be extended betweenthe stations is less than the number of load devices to be operated.`More particularly, the invention relates to apparatus of the abovecharacter in which a control voltage at the operating station varies inaccordance with changes in the value of resistance effective across theconductors at the operating station ends thereof as determined bymanipulations of the control elements at the control station and inwhich such control voltage is applied to a mechanism operable toenergize the respective load devices individually in response todifferent predetermined values of the control voltage.

One object of the invention is to provide, in apparatus of the abovecharacter, novel Voltage responsive "mechanism which is simpler inconstruction and operation and requiresV fewer parts lthan similar priorart mechanism.

Another object is to provide novel voltage responsive mechanism in whichenergization of the respective load devices is controlled by acorresponding number of electron tubes and in which the grid biascircuits of the latter are composed of elements of fixed characteristicsand are arranged in a novel manner such that dilferent predeterminedvalues of the control voltage are utilized to condition the respectivetubes for conduction individually and, at the same time, to maintain allof the tubes other than the selected one in a cut-off condition.

A more detailed object lis to arrange the bias control circuits of aplurality of double grid load control tubes in a novel manner forapplication of the control voltage to both grids of allof the tubessimultaneously to con dition a different tube for conduction whilemaintaining y the other tubes cut od in response to each predeterminedvalue of the control voltage.

The invention also resides in the novel arrangements of the load devicesand tubes at the operating station to insure that a single load deviceis energized in response to the simultaneous manipulation of more thanone control element.

Other objects and advantages of the invention will become apparent fromthe following detailed description taken in connection with theaccompanying drawings, in which- Figure l is a schematic view and wiringdiagram of control apparatus embodying the novel features of the presentinvention.

Fig. 2 is a chart showing the bias voltage on the grids of the controltubes under dilerent operating conditions.

In the drawings, the invention is shown for purposes of illustrationembodied in electrical apparatus for controlling the selectiveenergization of a plurality of load devices Ll, L2, L3, L4, LS, L6, L7and LS at an operating station 10 in response to manipulation of controlswitches S2, s3, s4, ss, s6, S7, and ss at a remoteryfldatd sta- Cil-circuits by which the control voltage is applied t tiddid ire tion l1.The improved apparatus is of vthe type in w" ch the lswitches cooperatewith a plurality of resistorsl, R3, R4, R5, R6, R7, and R8 at thecontrol statio change the value of the resistance eective acrossyconduc-tors 12 and i3, herein two, fewer in number thanlthe loaddevices, extending between the stations. e

series with a constant voltage direct current source" across theconductors l2 and 13 so that. variations i' eiective resistance acrossthe latter pro-duce corresponding changes in the current flow throughthe resistancetel ment and in the value of the voltage across the lat r.rhis voltage is a control voltage and is applied i o" voltage responsivemechanism 16 which is operableifto energize the load devicesindividually in responsefit'o different predetermined values of thecontrol voltage.i{

The present invention contemplates the provisiornlfnv apparatus of theabove character, of a novel Voltagetr'esponsive mechanism 16 which, ascompared to similar prior art apparatus, requires fewer parts and issimpler in construction and operation. Generally, the improved Voltageresponsive mechanism comprises a sensingsisection 17 responsive to thecontrol voltage across thev'resistance element 14 and having differentpredetermined output voltage conditions corresponding to the respectivepredetermined control voltage values. Such output voltage conditions aresensed by a series of load control elements Tfr, T2, T3, T4, T5, T6, T7,and T8 whichtco'rrespond in number to the load devices and are operablerespectively in response to the different predetermined voltageconditions to energize the corresponding i-load devices individually. Inthis instance, the load control elements are electron tubes of the typehaving two control electrodes or grids both controlling conductionglftiythe tube and operable to render the tube non-conductive when either gridis biased negatively with respect t@ the cathode. The sensing section17, where such tubes,on stitute the load control elements, comprisesbias cogtrpl grids of all of the tubes simultaneously to conditioniadifferent tube for conduction while maintaining theother tubes cut olfin response to each predetermined va i i the control voltage. With thisarrangement, the ent predetermined output voltage conditions of the sening section referred to above are different bias voltage, which areapplied to the grids of the control tubes which change in accordancewith variations in theA p trol voltage to condition the tubes I forcoriduc on individually.

Each of the load tubes comprises output elect de plate P and a cathodeC, and the two grids G" and is conditioned for conduction when the pntial of both grids is substantially zero or of a positiver/,allie Toutilize the control voltage of the resistancejfelehdlit lili to changethe potential of both grids of all offth control tubes simultaneously,the resistance e coupled to each of two bias control circuits. 4 ylatter is connected to one grid G of each tub operable to applypotentials of progressively magnitudes to the different grids. Theother' connected to the other grid G of each tube ,l erable to apply tothe different grids potential correspond to the potentials of the tirstcirc crease progressively in magnitude. n ,e ment 1d is coupled betweensuch circuits and vonducltotr lh common to the cathodes C of all of theub that changes in the voltage of the resistanceel f` applied inopposite directions to the two bial are added algebraically to thepotentials off, bringing the potentials of both grids of af: i, of thetubes approximately to zero While .rntain ng@ cut-ot potential on onegrid of all of the otherutubes.

nals of the divider resistors. vider resistor rl is connected betweenthe grid Gl of the Herein, each of the'load Vtubes' is a thyratron ofthe 2D21 type in which the cut-ott condition depends on the bias appliedto both the shield g'fr'id G and the control grid G. For example, whenthevoltageinthe plate circuit of such a tube is 150 volts"alte'rnating'current of 60 cycles per second, the tubesis lcut-ott by,a)negative bias voltage ot A2 volts on the shield grid G when the controlgrid bias r3, r4,MrS,;/r6, and r/ connected in series with a resistor.1.9 acrossa suitable `constant voltage direct current source 2,0, thepositive terminalv of the latter being connected through the Aresistor19 and a conductor 21 to the first dividerrl and theA negative terminalVbeing connected by a conductor 2227to the last divider resistor 1'7. The,divider resistors are .connected between adjacent ones of the controlgrids G of the series of load tubes or, stated vanother way, the.lrespective grids G are connected to successive taps of the divider orto corresponding termi- For example, the lirst di first tube T, and thegrid G2 of the second tube T2, the grids G1y and G2 being connected tothe last two taps at` one end of the divider. Herein, the voltage of thesource 2.0 is,` 75 volts and the values of the divider resistors'and theseries resistor 19 krespectively are 500 ohms each yand 5900 ohms sothat the voltage drop across each divider 'resistor is approximately 4volts and the total drop across the divider from the iirst grid G1 tothe last grid GS is a negative 28volts.

The second'bias circuit comprises a similar divider having 'resistorsr1', r2', r3', r4', f5', r6', and r7 connected respectively betweenadjacent ones of the other grids VG of the load tubes and in series witha resistor 23 across a suitable constant voltage direct current source24. The positive terminal of th'ewlatter is connected through theresistor 23 and a conductor 24 to the last divider resistor k7 and thenegative terminal is connected by a Aconductor 25 to theiirst resistorr1. The values of the source `voltageand of the divider resistors arethe same vas in the Iiirst bias circuit, butfthe value of the seriesrevolts positive when'thev grid G1 of the rst `tube is negative. Aresistor` 26 of suitableV value, for example, 2 million ohms, isconnected in series with each grid to limit current flow therethrough.

To complete a bias circuit between each of the grids G and G and thecathode C of each tube T, opposite ends of the resistance element 14 areconnected respectively to the conductors 21 and Z5 and through thelatter to the iirst resistors r1 and r1 of the voltage dividers and anintermediate tap of the resistance element is connected through groundto the common cathode conductor 13. The resistance element 14 thus isdivided into two sections Mrz and Mb, the former being common to thegrid-cathode circuit of each of the grids G of the tubes and the latterbeing common to the gridcathode circuit of each of the grids G of thetubes. For example, the

lbias `circuit, of the grid G3 of the tube T3 extends from rthe cathodeC3 to the grid through the cathode conductor` 1S, ground, the lirstresistance section 14a, the conductor 21 and the iirst divider resistorr1 and r2. The bias circuit ofthe grid G3 of the tube T3 correspond-4ingly extends through the cathode conductor lib, ground,

the second resistance section Mb, the conductor 25, and the seconddivider resistors r1 and r2. In each of these circuits, the potential ofthe effective Section of the reresistance element 14 is addedalgebraically to the potentials of the effective divider resistors todetermine the potential of the grid with respect to the cathode.

The polarity of the source 15 connected to the resistance clement issuch that the potential of each of the sections llla and 14h of thelatter is opposed to that of the divider resistors in circuit withthesection. Herein, the resistance of the irst section 14a equals 1600 ohmsand that of the second section 14b equals 3200 ohms so that the voltagedrop across the latter is double that across the first section for agiven current iiow through the sections. The potential of the source 15is 105 volts and the negative and positive terminals of the source areconnected respectively to the second resistor section 14h and throughthe control station conductors 12 and 13 to the first resistor section14a.

When an open circuit exists across the conductors 12 and 13 at thecontrol station, no current flows through the grid control resistanceelement V14 and the only potentials applied betweenthe respective gridsand their corresponding cathodes are those applied by" the voltagedivider resistors. Under these conditions, as shown in the iirst line ofthe table in Fig. 2, the potential of each of the grids GliA and G1 withrespect to the cathode `C1 is zero and thetube T1 thereby is conditionedfor conduction when its plate or output circuit is closed. In the caseof all of the other'tubes, a potential of at least 4 volts negative isapplied to one of the grids of each of the 'tubes to maintain the latterin cut-ott condition. Since the tube T1 is conditioned for conductionwhen the conductors 12 and 13 are open-circuited, no switch is providedat the control station for this tube. Conduction nresistance of theproper value to provide the correct voltage drop across the resistanceelementlMjfor bringing the potential of each of the grids ofthecorresponding tube substantially to zero and thereby conditioning thetube for conduction. n

The resistors R2, R3, R4, R5, R6, R7, and R8 and the switches S2, S3,S4, S5, S6, S7, and S8 of the control station 11 also are arranged in anovel manner to insure that only one of the load tubes is conditionedfor conduction even though two or more of the switches vmay be closedsimultaneously. Such arrangement comprises connection of all of theresistors inseries with one conductor 12 and location of the, switchesin individual short circuits corresponding ,innu'rnber'to the resistorsand extending between corresponding ends of the latter and the otherconductor 13. Thus, when any switch such as the switch S7 is closed,Vthe, resistors R7 and R8 located between the switch and thetirstconductor 12 are connected across the conductors and the remainingresistors R2, R3,

R4, R5, and R6 are shortcircuited so that closure of any of the switchesS2, S3, S4, S5, and S6 associated with such remaining resistors does notchange the value of resistance acrossthe conductors. In the presentinstance, the switches S3, S4, SS, S6, S7, and S8 are of the momentarycontact push-button type which are spring urged into'open positions. Fora purpose to appear later, the switch S2 is of the single pole singlethrow type. The values of the resistors R2 to R8, in this instance, are21,000 ohms, 7000 ohms, 3500 ohms, 2100 ohms, 1400 ohms, 1000ohms, and1200 ohms respectively.

Each of the load devices L1 to L8, which herein are relays, is arrangedto be energized in response to con duction by the corresponding one ofthe load tubes T1 to T 8. For this purpose, the coil of the relay isconnected in series with the plate P in the plate or output circuit Aof-the corresponding tube.

Vto the plate circuits of all of the load tubes and is connected to asuitable alternating current power supply 29 series with normally opencontacts l connectedto the common cathode` conductor 13 through ground.Herein, the potential of this source is 15() volts. With such analternating source in the plate circuit of each load tube, the latterconducts only in positive half cycles and is rendered nonconductive inthe first negative half cycle after a cut-oli potential is applied toone of its grids. To avoid energization of each load relay in the eventthat the associated tube is rendered conductive for only a few'cycles ofthe source 29 in response to transient currents in the grid circuits,the relays preferably are of the time delay type Well known in the artand requiring current flow therethrough for a longer period such as tencycles before pulling in. t

It will be seen from the foregoing that each load tube conducts toenergize the associated load relay when the plate circuit of the tube iscompleted and the potential of each grid of the tube is substantiallyzero. To avoid conduction of more than one tube and energization of theassociated load devices simultaneously, contacts of the load relays L1to L8 and the output circuits of the tubes are arranged in a novelmanner such that the output circuits of all of the tubes disposedbetween a terminal one of the tubes and all tubes between the terminaltube and any of the tubes which is conducting are disabled orinterrupted upon energization of the relay associated with theconducting tube. Such operation is accomplished by connecting the commonplate conductor 23 directly to the conductor 27 of the last load relayL8 and by including, in the output circuit of each tube starting withthe rst tube T1 and between the load relay coil thereof and the commonplate conductor, normally closed contacts of each of the load relays ofall of the tubes up to the last tube T8. ln the present instance, a pairof normally closed contacts of each load relay starting with the lastrelay LS is connected between the conductor 27 of the latter and theadjacent relay coil conductor 27 so as to be included in series in theoutput circuits of all of the tubes disposed between the correspondingtube and the iirst tube T1. Thus, the output circuit of the fourth tubeT4, for example, includes in series in its output circuit the normallyclosed contacts LS1, Lol, L71 and L81 of the respective relays LS, L6,L7, and L8 so that, when any one of the latter is energized, the fourthtube is disabled.

In the use of a control of the above character, other contacts such asthe contacts Lil?. to L32 of the respective relays Ll to L8 may beutilized in suitable control circuits to perform different functionsupon energization of the relays. It is sometimes desirable to use two ofthe load relays to control the same condition, for example,

direction of rotation of a spindle in a machine tool, and

to maintain the condition as established by energization of either relayeven after the latter is deenergized. This is accomplished in thepresent instance by the provision of a control relay AR whose coil isconnected in relay L1 and normally closed conta LEE of the second relayL2 across a suitable voltage source 33 so that the control relay circuitis closed and opened respectively upon energization of the first relayLl or the second relay L2. The relay AR includes normally open contactsARI which are connected in a shunt around the first relay contacts Lilto complete a holding circuit for the control relay when the rst relayis deenergized. To avoid continued energization of either load relaywhile the condition to be established by its energization is alreadyprevailing, normally closed contacts ARZ and normally open contacts ARTof the control relay are connected in series respectively with the irstload relay coil `and the second load relay coil. The switch S2controlling the second load relay L2 is a single pole single throwswitch to enable the operator at the control station to leave the switchactuator in one position or the other `and thereby determine byobservation what condition of the control relay prevails. The conditionsuch as direction of spindle rotation which is controlled in accordanceor" the iirst load i i with euergization and deenergization of thecontrol re= lay AR may be changed in response to opening and closing ofanother set of contacts (not shown) of this relay. In the operation ofthe improved control apparatus described above, let it be assumed thatpower is available at the various voltage sources and that all of theswitchesv S2 to S8 are open. Referring to Fig. 2, it will be seen that,under these conditions, the grids G1 and G1 of the first load tube T1are both at zero potential so that the tube is conditioned forconduction. Oneof the grids of each of the other tubes T2 to T8 is at anegative potential of at least 4 volts and the other tubes are therebymaintained in cut-oliF condition so that the normally closed contactsL21 to L81 in the output circuit of the rst tube T1 are closed. With the.control relay AR deenergized so that its contacts ARZ are closed, theplate circuit of the first tube is completed and the latter conducts forenergization of the first load relaylLl. The contacts L11 of this relayclose to complete the energizing circuit of the control relay AR. Uponpull-inl of the latter, the contacts ARS thereof in the plate circuit ofthe second tube close to enable this tube to conduct when the switch S2is closed, the contacts ARZ open to disable the rst load tube outputcircuit, Aand the contacts ARl close to complete the hold-ing circuit ofthe control relay. The energized condition of the control relay prevailsduring closure of the other switches S3 to S8 and until the switch S2 isclosed because the holding circuit for the control reiay does notinclude any of the contacts L3 to L31 ofthe relays L3 to L8.

When the switch S2 is closed, all of the control station resistors R2 toR3 are connected in series across the conductors 12 and i3 to complete acircuit through the grid control resistance element 14 and the source l5for producing a potential of 4 volts across the rst section Mrz inseries with the resistors ri to 17 of the iirst bias circuit, therebyraising the potential of each of the grids Gl to G7 by this amount. Atthe same time, a potential or" S volts negative across the secondsection ld!) of the resistance element is applied in series with theresisters r1' to r7 to lower the potentials of the grids G1 to G7 of theload tubes. As a result, the grids G2 and G2' of the second tube T2 areboth at Zero potential and the tube conducts to energize thecorresponding relay L2 and open the circuit to the control relay AR atthe contacts L22. The control relay then is deenergized and the contactsARZ thereof close in the plate circuit of the iirst tube T1 to enablethe latter to conduct when the switch S2 is opened. Also, the contactsARS open in the plate circuit of the second tube to disable this tube.This condition of the control relay prevails during closure of any ofthe other switches S3 to S8 and until the switch S2 is opened.

Upon closure of any of the other switches S3 to S8, a proper voltage isapplied across the resistance element 14 and in the respective biascontrol circuits to bring the potentials of both grids ofthecorresponding load tube substantially to zero for conduction by the tubeand ener gization of the lcorresponding load relay in a manner similarto energization of the second load relay L2 upon closure of the switchS2. As soon as all of the switches are opened, a negative bias isrestored to one grid of each of the tubes except the first tube T tomaintain such tubes nonconductive. If two or more switches are closedsimultaneously, only the corresponding tube nearest the power supply endof the series of tubes will conduct. Thus, closure of the switches S2and S7 (see Fig. 2) corresponding to the second and seventh tubes T2 andT7 results in conduction only by the seventh tube T7 due toshort-circuiting of all control station resistors eX-cept R7 and R8 anddue to opening of the normally closed contacts L71 in the plate circuitof the second tube. Similarly, closure of the switches S4 and S7 for thefourth and seventh tubes T4 and T7 results in conduction only by thelatter tube. It' the last `switch S8 is closed along with the switch S7for the seventh tube, then only the eighth tube T8 will conduct.

By virtue of the novel arrangement of the load tubes and the biascircuits-described above, a single control voltage may be utilizedy notlonly to condition the tubes for conduction selectively, butalso tomaintain in a cutoil condition the other'tubes' which are not selected.Such arrangement alsofmakes it possible to use only two conductorsbetweenthe operating station and the control station and to locateatfthelatter 'only resistance elements and switches which' are-ruggedand durable and whose characteristics remai'nptined in service use. Thesize of the control station andthe number of parts required to belocated there thus are reduced and the portability of the station isincreased. Due to the man lner of connection of the switches S2 to S8 tothe resistors R2 to RS at the control station and of the normally closedcontacts of the load relays into the output circuits of the load tubesTl to Td, energization of two load relays simultaneously is avoided eventhough the operator may inadvertently close more than one of theswitches at the same time.

l claim as my invention:

l. In control apparatus, the combination of, a plurality of controlelements arranged in a sequence and cach having an output circuit, acorresponding number of relays arranged in a similar sequence andrespectively connected in the different output circuits of said controlelements for energization of each relay when the corre sponding controlelement is rendered effective, each of said relays having normallyclosed contacts in the output circuit of a preceding control element`whereby energization of one of said control elements and its relayresults in interruption of the output circuit of such preceding controlelement, selectively operable means for rendering said control elementseffective individually, and a separate relay controlled by the terminaland adjacent ones of said relays at said one end of said sequence andhaving an energizing circuit including normally closed contacts of theadjacent relay and normally open contacts of the terminal relay forenergization and deenergization of the separate relay in response toenergization of the terminal relay and the adjacent relay respectively,said separate relay having normally open contacts connected in parallelwith said normally open terminal relay contacts to yprovide a holdingcircuit for the separate relay and normally closed contacts and normallyopen contacts con nected respectively in series with the coil of saidterminal relay and the coil of said adjacent relay whereby to enable acondition established by selective energize- Vtion of either of theterminal and adjacent relays to be maintained after deenergizaion ofsuch relay as occurs upon energization of any of the relays locatedbetween each `ot theiterminal and adjacent relays and the other end ofsaid sequence.

2. ln control apparatus, the combination of, a plurality of controlelements arranged in a sequence and each having an output circuit, acorresponding number of relays arranged in a similar sequence andrespectively connected in the different output circuits of said controlelements for energization of each relay when the corresponding controlelement is rendered eltective, each of said relays having normallyclosed contacts in the output circuit of a preceding control elementwhereby energization of one of said control elements and its relayresults in interruptionot' the outputv circuit of such preceding controlelement, selectively operable means for rendering said control elementseffective individually, and a separate relay controlled by the terminaland the adjacent ones of said relays at said one end of said sequenceand having an energizing circuit including normally 'closed contacts ofa first one of the adjacent and terminal vrelays and normally opencontacts of the other of such relays for energization landdeenergization of the separate -relay in response respectively toenergization of thel "s first relay and deenergization of. the otherrelay, saidv separate relay having. normally' opencontactsin parallelrwith vsaid normally open contacts: ofasaidtirst relay :whereby to enablea conditionestablished byselec'tive energization of either of theterminal and adjacent relays topbe maintained after deenel'rgization ofsuch relay as occurs upon energization of `any of' thefrelays locatedbetween each of the terminal and adjacent relays Vand the other end ofsaid sequence.

3. ln control apparatus, the combination of, a power supply, a pluralityof load control tubeseach having two output electrodes and an outputcircuit connected between said electrodes and including said powersupply, a plurality of load relays corresponding in number to said tubesand respectively connected in the different output circuits of thelatter in series with one of said. electrodes thereof for energizationof each relay when theoutput circuit of the corresponding tube iscompleted and the .tubeis conditioned for conduction, each of saidrelays having normally closed contacts in the output circuits Otan endone of said series of tubes and all tubes in the seriesbetween such endtube and the tube corresponding to the relay whereby the output circuitof each tube is interrupted by conduction of any of the tubes locatedbetween such tube and the other end tube of the series and ienergizationof the associated relay, and selectively operable biasing means inaddition to said normally closed relay contacts for conditioning aselected one of said tubes .'or conduction while the other tubes arebiased non-conductively.

4. in control apparatus, the combination of, afpower supply, a pluralityor load control tubes each having two output velectrodes and an outputcircuit connected between said electrodes and including said powersupply,

Va plurality of load relays corresponding innumber to the tube isconditioned for conduction, the output circuit of each tube includingbetween the tube and said power supply, normally closed contacts of eachof said relays in the output circuits ot the other of said tubesdisposed between such tube and one end of said series whereby conductionby any tube and energization of the corresponding relay results ininterruption of the output circuits of the tubes between the conductingVtube and the other end ot the series, and selectively operable biasingmeans in addition to said normally closed contacts for conditioning aselected one of said tubes for conduction while the other tubes arebiased non-conductively.

5. in control apparatus, the combination of, a plurality of load controlelements arranged in a series and each having an output circuit, aplurality of load devices corresponding in number to said controlelements and respectively connected in the dilerent output circuits ofthe latter for energization of each load device `when the correspondingcontrol element is rendered effective, each of said load devicesoperating when energized to interrupt the output circuits and therebydisable the corresponding load devices of the control element at one endof said series and all ot the control elements between such end elementand the control element corresponding to the energized load devicewhereby to avoidV actuation of more than one load device simultaneously,and selectively operable biasing means in addition to said load nectedto said first electrodes and operable to apply bias potentials ofprogressively increasing magnitude thereto, second bias control circuitsconnected to said second electrodes and operable to apply biaspotentials of progressively decreasing magnitudes thereto, and a circuitfor changing the bias potentials of said electrodes selectively andbringing the potentials of the electrodes of any one of said tubessubstantially to zero to condition the tube for conduction whilemaintaining a cut-off potential on one of the electrodes of each of theother tubes, said circuit including two conductors extending betweensaid stations, two voltage control elements connected respectively tosaid first and second bias circuits to vary said potentials in thelatter in accordance with voltage changes across the elements, a sourceof voltage connected in series with said elements across said conductorsat said operating station, and remote control resistance means at saidcontrol station bridging said conductors and selectively adjustable tovary the voltage across said elements.

7. In control apparatus, the combination of, a plurality of load controltubes arranged in a series and each having a cathode and first andsecond control electrodes, a conductor connected to the cathodes of saidtubes, a plurality of first resistors connected in series andrespectively connected between adjacent ones of said first electrodes ofsaid tubes in said series, a second resistor connected between saidcathode conductor and one terminal of said series of said firstresistors to complete a bias circuit between the cathode and the firstelectrode of each tube, a plurality of third resistors connected inseries and respectively connected between adjacent ones of said secondelectrodes of said tubes, a fourth resistor connected between saidcathode conductor and one terminal of said series of third resistors tocomplete a bias circuit between the cathode and the second electrode ofeach tube, means including a voltage source connected across each ofsaid series of first and third resistors to apply to said firstelectrodes potentials of progressively increasing magnitudes withrespect to the associated cathodes and to apply to said secondelectrodes potentials of progressively dee creasing magnitudes withrespect to the associated cathodes, and a control circuit including saidsecond and fourth resistors and selectively operable electrical means tochange the potentials across the second and fourth resistors andcorrespondingly change the potentials effective on said electrodes ofsaid tubes.

8. Control apparatus having, in combination, a plurality of load controltubes arranged in a series and each having a cathode and first andsecond control electrodes, a first bias control circuit comprising adirect current source of voltage and a plurality of resistors connectedin series across said source and having corresponding terminalsconnected to the respective first electrodes of said tubes to bias theseelectrodes at different potentials of progressively increasingmagnitude, a second bias control circuit comprising a direct currentsource of Voltage and a plurality of resistors connected in seriesacross the source and having corresponding terminals thereof connectedto the respective second electrodes of said tubes to bias theseelectrodes at potentials of progressively decreasing magnitudes, aconductor connected to said cathodes of said tubes, two resistors, oneconnected between said cathode conductor and each of said bias circuitsat points in the latter between said sources and said resistors thereofto complete bias circuits between the cathode and each of saidelectrodes of each tube, and a third circuit including means for varyingthe voltage across said two resistors selectively to increase saiddecreasing potentials and decrease said increasing potentials to bringthe potentials of both electrodes of each of said tubes substantially tozero to condition the tube for conduction.

9. In control apparatus, the combination of, a first grid bias circuitincluding a voltage divider having a succession of taps of progressivelyincreasing potentials, a

,second grid bias circuit including a Voltage divider having asuccession of taps of progressively decreasing potentials correspondingto the taps of said first divider, a series of electron tubes eachhaving two control electrodes one connected to a tap of said firstdivider and the other con-- nected to a corresponding tap of oppositepotential on said second divider to render the tube conductive when thepotentials of both taps are approximately zero and to maintain the tubenon-conductive when the potential of each tap is below the grid cut-offpotential of the tube, a conductor connected to the cathodes `of saidtubes, a bias control resistor having a first section connected inseries with said first divider between the latter and said conductor anda second section connected in series with said second divider betweenthe latter and said cathode conductor, and a third circuit extendingacross said control resistor and including adjustable means operable tovary the current ow through the resistor selectively in a direction toincrease the potentials of said rst divider taps and decrease thepotentials of said second divider taps to render said tubes conductiveselectively.

10. In control apparatus, the combination of, a plurality of loadcontrol tubes arranged in a series and each having first and secondcontro-l electrodes, a first bias control circuit connected to saidfirst electrodes of said tubes to apply to such electrodes differentpotentials increasing progressively in magnitude from one end of saidseries to the other, a second bias control circuit connected to saidsecond electrodes of said tubes to apply to such electrodes differentpotentials corresponding to the potentials of said first circuit butdecreasing progressively in magnitude from said one end of said seriesto said other end thereof, and a third bias control circuit including anelement connected to said first and second circuits and having apotential which is added algebraically to said potentials of the twocircuits, and selectively variable means for adjusting the potential ofsaid third control circuit element to increase said decreasingpotentials and decrease said increasing potentials to bring thepotentials of both electrodes of any one of said tubes substantially tozero and thereby condition the tube for conduction while maintaining anegative cut-ofi potential on one of the electrodes of all of the othertubes.

ll. ln control apparatus, the combination of, a plurality of loadcontrol tubes arranged in a series and each having first and secondcontrol electrodes and adapted to conduct current when the potentials ofboth electrodes are of substantially the same predetermined value, firstbias control means connected to said first electrodes and operable toapply thereto bias potentials of progressively increasing magnitudeslarger than said predetermined value, second bias control meansconnected to said second electrodes and operable to apply thereto biaspotentials of progressively decreasing magnitudes larger than saidpredetermined value, and third bias control means connected to saidfirst and Second means to change the potentials applied by the latter tosaid electrodes, and selectively variable means for adjusting said thirdbias control means to increase the decreasing potentials whiledecreasing the increasing potentials by corresponding amounts whereby tobring the potentials of both electrodes of any one tube substantially tosaid predetermined value and thereby condition the tube for conductionwhile maintaining a negative cut-oii potential on one of the electrodesof all of the other tubes.

l2. In control apparatus, the combination of, a serie! of load controlelements each `responsive to two different bias potentials and renderedoperative when such potentials are of substantially the samepredetermined value and inoperative when either potential is of amagnitude higher than the predetermined value, first bias controlcircuits normally applying to said elements bias potentials havingmagnitudes larger than said predetermined value to maintain the elementsinoperative and increasing progressively from one element to the next inthe series, second bias control circuit normally applying to saidelements bias protentials having magnitudes larger than saidpredetermined value and decreasingy progressively from one element tothe next inthe series, and third bias control circuits connected tovsaidr'st and second circuits to change the eiective `biaspotentials onsaid elements, and selectively variable means foradjusting said thirdbias control circuits to increase the decreasingpotentials Whiledecreasing the increasing potentials by corresponding amounts whereby tobring nothpotentials applied to any one element substantially tosaidpredetermined value and thereby render the element operative whilemaintaining one of the potentials on each of the other elements higherthan the predetermined value to render the other elements inoperative.

References Cited-'in thellle of this ,patent UNITEDS STATES PATENTSSprague 'Ily 27, Buyk'o Apr. 21, Francis Aug. 18, Demarest Jain.l 8,Sarbey July 16, Morehouse et al. Oct. 20, Lowell July 22, Pouliart June29, Oberman June 12,

Oberman et al Aug. 16,

